Recently, nickel hydride storage batteries and lead storage batteries are known to be used as power storage devices such as secondary batteries. In many cases, these power storage devices are required to be small-sized due to downsizing of mobile devices, limitations of installation space, and the like. Accordingly, attention is being paid to lithium-ion batteries, having high energy density. As a packaging material (hereinafter, also referred to simply as a “packaging material”) used for a lithium ion battery, although a metallic can has been widely used, a multilayer film that is light, has high radiation performance, and can be applied at low cost has come to be increasingly used.
An electrolytic solution of a lithium ion battery is composed of an aprotic solvent such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate or ethylmethyl carbonate, and an electrolyte. A lithium salt such as LiPF6 or LiBF4 may be used as the electrolyte. However, these lithium salts may generate hydrofluoric acid due to a hydrolysis reaction with water. Hydrofluoric acid causes corrosion of the metal surface of a battery member and a decrease of the laminate bond strength between layers of the packaging material formed of the multilayer film.
Accordingly, a barrier layer formed of a metal foil such as an aluminum foil is provided inside a packaging material formed of a multilayer film in order to prevent water from entering through the surface of the multilayer film. For example, there is a known packaging material in which a base layer having heat-resistance/a first adhesive layer/a barrier layer/a corrosion protection layer which prevents corrosion due to hydrofluoric acid/a second adhesive layer/a sealant layer are sequentially laminated. The lithium-ion battery which uses the above-described packaging material is also referred to as an aluminum laminate type lithium-ion battery.
As one type of an aluminum laminate type lithium-ion battery, a lithium-ion battery is known in which a recess is formed on a part of the packaging material by cold forming, the battery contents such as a positive electrode, a separator, a negative electrode, an electrolytic solution, etc., are accommodated in the recess, and the remaining portions of the packaging material are folded and the edge portions are sealed by heat-sealing. Such a battery is also referred to as an embossed type lithium ion battery. Recently, for the purpose of increasing the energy density, an embossed type lithium-ion battery having recesses on both sides of the packaging materials to be bonded together has been produced. This type of lithium-ion battery can accommodate more battery contents.
The energy density of the lithium-ion battery increases as the depth of the recess formed by cold forming increases. However, pinholes or breaking readily occur during forming of the packaging material as the formed recess becomes deeper. Accordingly, a stretched film is used for the base layer of the packaging material to protect the barrier layer (metal foil). As described above, the base layer is typically joined with the barrier layer via an adhesive layer (for example, refer to PTL 1).